Permeability and elastic wave velocities in sandstone under hydrostatic pressure

Abstract

It is essential to investigate the influence of pores and fractures on physical and transport properties of rocks for understanding many key problems in seismology, volcanology and rock engineering. In rocks, water is primarily stored in and migrates through networks of pores and fractures at all scales. It is therefore important to know how fluid flow in such pores and fracture networks responds to the elevated pressures found at depth. Here, we report results from an investigation of changes in fluid permeability, and associated changes in P-wave and S-wave velocities, at elevated effective pressure for both intact and macro-fractured sandstone samples. Specifically, we used two types of sandstone; Clashach sandstone from Scotland and Shirahama sandstone from Japan, which have different porosities and pore size distributions. Wave velocities increased with increasing pressure for all cases. Additionally, wave velocities in macro-fractured samples were lower than those in intact samples. For Shirahama sandstone, permeability decreased with increasing pressure. By contrast, permeability of Clashach sandstone remained essentially constant over the whole pressure range. This lack of change is interpreted as being due to the presence of a well-connected network of large pores with high aspect ratios. It was also found that the difference in permeability between intact and macro-fractured sandstone samples was small. From these results, it is concluded that the networks of pores control the permeability in sandstone. © 2012 The Society of Materials Science, Japan.